(a) Field of the Invention:
This invention relates to a microscopic photographing apparatus wherein a proper exposure can be obtained for a minute sample image in photographing in a dark field or photographing an image obtained by a fluorescent microscope.
(b) Description of the Prior Art:
An average-spot light measurement switching system whereby the light of the part of 30% of the area of the picture frame (24.times.36 mm) of a 35 mm film or 1% of the center part of the picture frame is measured is adopted in most of photographing apparatus adapted for a dark field in which the background is dark or a fluorescent microscope among photographing apparatus for microscopes. In such apparatus, a mark showing a 1% light measuring area of a light receiving element is indicated within a viewer so that, by the user, an image may be superposed on the area and, in case the image occupies an area more than 1%, the image will be photographed without correcting the exposure but, when the sample image is less than 1%, the ratio of the area of the image to the area of the 1% area will be determined by sight and the exposure will be corrected. In some case, the light measuring area is made smaller to be a 0.25% light measuring area than the 1% light measuring area.
An example of this kind of conventional microscopic photographing apparatus is shown in FIG. 1.
That is to say, in FIG. 1, the reference numeral 1 represents an automatic exposure photographing apparatus body. A primary image A.sub.1 projected by an objective lens not illustrated is magnified by a photographing lens 2 and enters a prism 3. Here, by a first beam splitter 3a, a part of the light is transmitted to a light measuring optical system, passes through an image forming lens 4 and an average-spot switching slider 5 and forms an image on a light receiving element 6a connected to a photographing exposure time controlling circuit 6. By a second beam splitter 3b, another part of the light is transmitted to an observing optical system and forms a secondary image A.sub.2 through an image forming lens 8 of a viewer 7. The secondary image A.sub.2 is magnified by an eyepiece 10 and is observed or focused. The light havng passed through the prism 3 passes through a shutter 11 and forms a secondary image A.sub.3 on a film surface 13 of a film cassette 12.
The pinholes 5a and 5b of the slider 5 are of hole diameters corresponding respectively to the respective areas of the average and spot light measurements of the photographed picture frame. The positions of these pinholes 5a and 5b must be in the positions conjugate with the areas of the average and spot light measurements.
However, the above-mentioned conventional apparatus has such problems as are mentioned below.
FIG. 2 is an elevation of a focus plate 9. The secondary image 14a is an image larger than the spot light measuring area 15. In such case, the image is focused with double cross lines 16 and then can be photographed without correcting the exposure. However, in case the secondary image is smaller than the spot light measuring area 15, when the back is dark but the sample is bright, the exposure must be corrected carefully. That is to say, if the sizes of the secondary images 14b, 14c and 14d are respectively 1/2, 1/10 and 1/20 the spot light measuring area and the brightness is the same as of the secondary image 14a, first of all, the measured light amount will reduce and therefore will exceed the limit of the light measuring capacity of the light receiving element 6a, a warning will be issued and the automatic exposure will be impossible. Second, even if it is within the light measuring capacity, the user will determine the area ratio by sight and will correct the exposure. However, in the case of the secondary images 14c and 14d, it will be difficult to prevent the exposure from being wrong.
______________________________________ Sample image 14a 14b 14c 14d ______________________________________ Rate of the area 100% 50% 10% 5% of the sample image to the spot area Exposure indicating 10 20 100 200 time seconds seconds seconds seconds Necessary exposure unneces- 1/2 1/10 1/20 correction sary Final exposure time 10 seconds ______________________________________
Thus, in the prior art, there is a problem that, even with an automatic exposure photographing device, the exposure must be corrected more complicatedly than in the manual exposure operation.
If the light measuring area is made smaller, in order to keep the light measuring ability in response to it, the light measuring device must be made high in the performance. However, the performance is limited and 1% light measurement has been general. If the light measurement of the area less than 1% is made so as to be possible, it will be difficult to maintain the light measuring area indication and the optical axial precision of the light measuring optical system. This has been also one of the reasons for making the 1% light measurement a limit. As one of the solutions of this difficulty, there is taken a system wherein the optical axes between the viewer for confirming the light measuring area and the box containing the light receiving element are adjusted with each other and are then fixed to eliminate the interchangeability.